The present invention relates to rotary fluid pressure devices of the type in which a gerotor gear set serves as the fluid displacement mechanism, and more particularly, to such devices which are provided with two speed capability.
Although the teachings of the present invention can be applied to devices having fluid displacement mechanisms other than gerotors, such as cam lobe type devices, the invention is especially adapted to gerotor devices and will be described in connection therewith.
Devices utilizing gerotor gear sets can be used in a variety of applications, one of the most common being to use the device as a low-speed, high-torque motor. One common application for low-speed, high-torque gerotor motors is vehicle propulsion, wherein the vehicle includes an engine driven pump which provides pressurized fluid to a pair of gerotor motors, with each motor being associated with one of the drive wheels. Those skilled in the art will be aware that many gerotor motors utilize a roller gerotor, especially on larger, higher torque motors of the type used in propel applications, and subsequent references hereinafter to "gerotors" will be understood to mean and include both conventional gerotors, as well as roller gerotors.
In recent years, there has been a desire on the part of the vehicle manufacturers to be able to provide both the low-speed, high-torque mode of operation, such as when the vehicle is at the work site, and also a high-speed, low-torque mode of operation, for when the vehicle is traveling between work sites. One possible solution has been to provide a gerotor motor having a two-speed capability.
Two-speed gerotor motors are known from U.S. Pat. No. 4,480,971, assigned to the assignee of the present invention and incorporated herein by reference. The device of the cited patent has been in widespread commercial use and has performed in a generally satisfactory manner. As is well known to those skilled in the art, a gerotor motor may be operated as a two speed device by providing valuing which can effectively "recirculate" fluid between expanding and contracting fluid volume chambers of the gerotor gear set. In other words, if the inlet port communicates with all of the expanding chambers, and all of the contracting chambers communicate with the outlet port, the motor operates in the normal low-speed, high-torque mode. If some of the fluid from the contracting chambers is recirculated back to some of the expanding chambers, the result will be operation in a high-speed, low-torque mode, which is the same result as if the displacement of the gerotor were decreased, but with the same flow rate through the gerotor.
In the two-speed gerotor motors which are in use commercially, and as shown in the above-cited patent, each volume chamber within the gerotor gear set has the opportunity to be a "recirculating" volume chamber, both as the volume chamber expands and as it contracts, while the motor is operating in the high-speed, low-torque mode. One result of each volume chamber being a recirculating volume chamber is a condition referred to as "oddly spaced" recirculating volume chambers which, it is believed, has led to an uneven torque ripple when operating in the high-speed, low-torque mode.
Accordingly, it is an object of the present invention to provide an improved two speed arrangement, especially suited for use with a gerotor motor, which will eliminate or substantially reduce the undesirable effects of the "oddly spaced" recirculating volume chambers, including reducing the unevenness of the torque ripple in the high-speed, low-torque mode.
As a result of the present invention, it is now understood that another disadvantage of the prior art two speed arrangements is that, in the prior art devices, all recirculating flow would have to pass through the commutating valuing. As is well understood by those skilled in the art, the fact that some fluid is recirculating in the high-speed, low-torque mode means that the total flow is substantially greater in the high speed mode. Unfortunately, in the typical, prior art arrangements, the addition of the two speed capability has resulted in valuing passages which are somewhat constricted in terms of flow capacity, by comparison to a conventional motor of the same speed and torque capacity. The result has been an undesirable increase in the pressure drop across the prior art two speed motors and, as is well known to those skilled in the art, the higher the pressure drop across a hydraulic motor, the less commercially desirable is the motor.
Accordingly, it is another object of the present invention to provide an improved two speed arrangement which does not require more constricted commutating valve passages, and therefor, does not result in an increased pressure drop across the motor.